Heretofore, combinations of transition metal compounds and aluminoxanes are known as highly active soluble catalysts for olefin polymerization (Japanese Patent Application Laid-open Nos. 19309/1983 and 217209/1985). Furthermore, it has been reported that ionic complexes are useful as active species of soluble catalysts for olefin polymerization [Journal of the American Chemical Society (J. Am. Chem. Soc.), Vol. 81, p. 81 (1959), Vol. 82, p. 1953 (1960), and Vol. 107, p. 7219 (1985)].
Moreover, examples in which these active species are isolated and applied to the olefin polymerization have been described in J. Am. Chem. Soc., Vol. 108, p. 7410 (1986), Japanese Patent Application Disclosure No. 502636/1989, Japanese Patent Application Laid-open No. 139504/1991 and EP Laid-open Patent No. 468651, and examples in which these active species are used together with organic aluminum compounds have been described in Japanese Patent Application Laid-open No. 207704/1991 and International Patent Laid-open No. 92-1723.
However, polymers produced by the use of complexes having cyclopentadienyl ligands which have been used in these conventional techniques have a problem that the molecular weight of the obtained polymer is low, in the case that the polymerization is carried out at a reaction temperature of 70 to 200.degree. C. which is efficient in an industrial process, or at a higher temperature.